Add tests

***The addition of tests is currently a work in progress.***

For a testing framework-like thing, I'm using [assert.sh][], [stub.sh][] and a custom [test.sh][] runner script. This setup worked out alright for stub.sh's own test suite, and another as of yet unreleased project of mine. I have also setup [Travis-CI][] to run all tests against Tmux versions 1.6, 1.7, 1.8, and 1.9a, which as of now I consider the officially supported Tmux versions.

[assert.sh]: https://github.com/lehmannro/assert.sh
[stub.sh]: https://github.com/jimeh/stub.sh
[test.sh]: https://github.com/jimeh/tmuxifier/blob/359f1a9a94c6b9f9507b78d0680bd22a170c438f/test.sh
[travis-ci]: https://travis-ci.org/jimeh/tmuxifier

### Todo

There's a decent number of things which are in need of tests. Below is an overview of them along with current test status.

- [x] Basic test framework setup.
- [ ] Main `tmuxifier` executable in bin folder.
- [ ] Files in lib folder:
    - [x] env.sh
    - [ ] layout-helpers.sh:
        - [ ] tmux
        - [ ] new_window
        - [ ] split_v
        - [ ] split_h
        - [ ] split_vl
        - [ ] split_hl
        - [ ] clock
        - [ ] select_window
        - [ ] select_pane
        - [ ] send_keys
        - [ ] run_cmd
        - [ ] session_root
        - [ ] window_root
        - [ ] load_window
        - [ ] load_session
        - [ ] initialize_session
        - [ ] finalize_and_go_to_session
        - [ ] __expand_path
        - [ ] __get_first_window_index
        - [ ] __go_to_session
        - [ ] __go_to_window_or_session_path
    - [x] runtime.sh
    - [x] util.sh
- [ ] Files in libexec folder:
    - [ ] tmuxifier-alias
    - [ ] tmuxifier-commands
    - [ ] tmuxifier-completions
    - [ ] tmuxifier-current-session
    - [ ] tmuxifier-edit-session
    - [ ] tmuxifier-edit-window
    - [ ] tmuxifier-help
    - [ ] tmuxifier-init
    - [ ] tmuxifier-list
    - [ ] tmuxifier-list-sessions
    - [ ] tmuxifier-list-windows
    - [ ] tmuxifier-load-session
    - [ ] tmuxifier-load-window
    - [ ] tmuxifier-new-session
    - [ ] tmuxifier-new-window
    - [ ] tmuxifier-resolve-command-path
    - [ ] tmuxifier-tmux
    - [ ] tmuxifier-tmux-version
    - [ ] tmuxifier-version